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Excessive redundancy is the scourge of efficiency
This year's annual Seminar on the work of the Systems Interface Committees (SIC), organised by the Railway Division of the Institution of Mechanical Engineers was especially illuminating and I was surprised that the great hall was not packed. Much more in future issues, but as this month Informed Sources is all about IEP, I must mention a thought provoking section in the presentation on the work of the Vehicle/Traction Supply SIC given by its Chairman Tony Mercado, the head of Alstom Train Care and master of all things Pendolino.
Tony's theme was energy saving and, of course, excess weight means energy wasted. ‘Why' he challenged ‘do we need fully-rated friction brakes on electric trains fitted with regenerative braking on the motored axles'.
‘Fully-rated' is defined as being able to handle normal and emergency braking requirements if the electric braking is switched off. Absorbing all the braking energy all of the time means larger and heavier discs and pads. Heavier discs also mean more rotational energy to be created and dissipated.
But in practice, the regen' brake handles the bulk of the braking load, returning 15-17% of the electric current consumption back into the grid. Friction braking is required only at low speed and for coming to a halt.
Chairman of the conference was another old engineering chum, Clive Burrows, DfT Rail's Director Technical & Professional. During his BR career he was involved with the Class 323 Electric Multiple Unit fleet which pioneered regenerative braking under the wires on BR.
When the fleet ran into problems with ice on the contact wire triggering interference protection, the regenerative braking was switched off while the software was re-written. According to Clive, during this period, the lack of electric braking reduced brake pad life from 18 months to 18 days.
Tony Mercado argues that since the full rating of the disc brakes is required only in the rare event of electric braking failure, there is no need to rate the brakes for continuous duty. Instead the brakes and discs should be rated for, say, two emergency stops from maximum line speed.
This is not a new idea. The tread brakes on the Class 91 were, effectively a two-shot emergency brake.
And if a take-off is aborted, the airliner does not taxi back and try again. It goes into the workshop for new pads and discs.
Similarly, if a train had to pull off a couple of emergency stops on friction brakes alone, which is unlikely, that would be it for the day. It would the next station at reduced speed and then into the depot.
Not requiring repeated full service braking would reduce the mass of both brakes and discc. But if the friction brakes are backing up regenerative braking, why have rheostatic braking too?
Rheo' braking is used only if the train can't regenerate into the contact wire. Instead the energy from the traction motors is dissipated as heat in resistors – giant versions of the elements in electric fires.
Here is more surplus weight being carried around. Getting rid of the rheostatic braking equipment would save around 500kg per traction package.
Obviously, there would have to be safety and operational assessments to see how often rheo' braking has to cut in and for how long the friction braking system would take over. But as energy efficiency rises up the agenda, this is the sort of question traction and rolling stock engineers will have to ask.